Shape-memory polymers (SMPs) are polymers that can switch between different shapes upon the application of an external stimulus, such as heat. For example, a thermal-responsive shape-memory effect can be achieved by cooling an elastically deformed material through a thermoreversible transition that bolsters its internal cohesive energy density, thereby fixing the temporary shape. Upon heating, stored elastic energy can be released and the material recovers its original shape, resulting in what is called one-way shape-memory (Anthamatten M et al. Macromolecules 2013, 46 (10), 4230-4234). In recent years, the engineering of SMPs has become incredibly sophisticated (see e.g., Xie T. Nature 2010, 464 (7286), 267-270; Luo Y W et al. Adv Mater 2013, 25 (5), 743-748; Luo X F and Mather P T. Adv Funct Mater 2010, 20 (16), 2649-2656; Torbati A H et al. Soft Matter 2014, 10 (17), 3112-3121; Chatani S et al. Macromolecules 2014, 47 (15), 4949-4954; Zotzmann J et al. Adv Mater 2010, 22 (31), 3424-3429). SMP action can now be triggered by various stimuli such as light, moisture, or applied magnetic fields (Huang W M et al. Appl Phys Lett 2005, 86 (11); Mohr R et al. PNAS USA 2006, 103 (10), 3540-3545; Lendlein A et al. Nature 2005, 434 (7035), 879-882). However, many potential applications (e.g., artificial muscles and actuators) are constrained by one-way shape-memory; in other words, many conventional SMPs do not return to their temporary shape upon re-cooling, and they must be reprogrammed each cycle.
Two-way shape memory is the ability to reversibly cycle between two different shapes. Strain-induced crystallization of polymers has been used to create two-way shape memory at different temperatures (Zotzmann J et al. Adv Mater 2010, 22 (31), 3424-3429; Chung T et al. Macromolecules 2008, 41 (1), 184-192). However, an external load is typically required to direct crystallization along a preferred direction. Other two-way shape actuators have been created by joining a layer of pre-elongated shape memory polymer with a layer of unstretched elastomer (Chen S J et al. Compos Sci Technol 2010, 70 (10), 1437-1443; Westbrook K K et al. Smart Mater Struct 2011, 20 (6), 065010). The resulting composites have built-in stress that directs crystallization in the shape memory layer, resulting in bending actuation upon thermal cycling. Behl et al. have demonstrated free-standing copolymer networks, with two phase-separated crystallizable domains, that are capable of reprogrammable and reversible bending (Behl M et al. Adv Mater 2013, 25 (32), 4466-4469). In their approach, one set of crystallizable domains determines the shape-shifting geometry while the other provides the thermally-controlled actuation capability.
Despite these advances new SMPs are still needed. For example, shape memory polymers that have two-way shape memory, even without external loads or the need for additional layers or phases, would be desirable. The methods and compositions disclosed herein address these and other needs.